As is known, in the field of storage systems there is a need to reach high storage capacities with a high data-transfer rate (bit rate) and a low error rate (bit-error rate), at the same time reducing manufacturing costs and area occupation.
Storage systems currently used are hard-disk drives (with miniaturized dimensions) and flash RAMs. In hard disks, a ferromagnetic material is used for storing the information in the form of bits, the state of magnetization of each cell determining the value of the bit stored.
These storage systems have intrinsic technological limits as regards the increase in the data-storage capacity and read/write speed, and the reduction of their dimensions. For example, in the case of hard disks, the so-called “superparamagnetic limit” hinders reduction in the dimensions of the magnetic-storage domains below a critical threshold.
Amongst the innovative solutions proposed, promising ones are storage systems using a storage medium made of ferro-electric material, in which reading/writing of individual bits is performed by interacting with the ferro-electric domains of the ferro-electric material. As is known, a ferro-electric material has a spontaneous polarization, which can be reversed by an applied electrical field. As illustrated in FIG. 1, this material has a hysteresis cycle in the diagram of the polarization charge Q (or, equivalently, of the polarization P) versus the applied voltage V. By exploiting this hysteresis cycle, it is possible to store the information in the form of bits. Without a polarization voltage imparted to the medium (V=0), two points of the diagram (designated by “b” and “e”) are in a stable-state and have different polarization, namely, of equal and opposite values. These points can remain in the stable state even for some years, thus maintaining the stored binary datum (for example, the point “b”, with positive charge +QH, corresponds to a “0”, whilst the point “e”, with negative charge −QH, corresponds to a “1”).
Reading circuits used in current data-storage systems, whether these are based on ferromagnetic or ferro-electric material (or other type of material), generally envisages the use of a transimpedance amplifier (TIA) for detecting an input current signal associated with the stored datum, and amplifying and converting it into an output voltage signal (to be used for subsequent processing). For example, in the case of ferromagnetic media, the input current signal is generated by an inductive sensor and is variable according to the magnetic field detected (and thus according to the stored datum); in more recent architectures, the input current signal is a function of the resistance of a read head moving over the medium, which again varies with the magnetic field. In the case of ferro-electric media, the input current signal is due to a variation of charge occurring in the ferro-electric material when a read voltage having a value higher than a threshold voltage is applied (known as coercive voltage, characteristic of the material, and designated by Vc in FIG. 1).
In detail, FIG. 2 shows a reading circuit 1, of a known type, coupled to a ferro-electric storage medium, represented schematically as a ferro-electric capacitor 2 with charge that varies according to the polarization condition (and thus to the stored datum), and having a first terminal 2a, connected to a reference potential (for example, to the ground of the circuit), and a second terminal 2b. The reading circuit 1 comprises: a voltage generator 3, configured to generate a read signal Vr; a transimpedance-amplifier stage 4 (of a known type, and not described in detail), connected to the ferro-electric capacitor 2 and to the voltage generator 3, and configured to detect and process a current signal caused by a charge variation ΔQ occurring in the ferro-electric material when the read signal Vr is applied; and a filtering stage 5 (of a known type, and not described in detail) connected to the output of the transimpedance-amplifier stage 4, and issuing an output signal (for example, an output-voltage signal Vout) as a function of the charge variation ΔQ.
The circuit described enables detection of the stored information: in particular, by applying the read voltage Vr, the capacitance of the ferro-electric capacitor 2 charges or discharges according to the previously stored information, and a correlated current flows between the storage medium and the reading circuit 1. The current is converted into voltage by the transimpedance amplifier, and processing of the output-voltage signal Vout thus generated (by an appropriate processing circuit, not illustrated) enables the determination of the value of the stored datum.
However, the circuit arrangement of the transimpedance amplifier (in particular, the presence of an RC group) limits the bandwidth of the reading circuit and consequently the data-transfer rate that can be achieved in reading and writing. In addition, the presence of passive components (e.g., a resistor) limits the performance of the reading circuit in terms of noise and power consumption, given that the bit-error rate depends in a known way on the signal-to-noise ratio.